(Whoa, two posts in one month, when was the last time I pulled that off?)
I don't expect this will be a very lengthy post, just an addendum to what I had to say in February about feeling the need to have everything I want present and accounted for. Much as this previously led me to spend a great deal of time and stress attempting to get the exact system I wanted, it also tends to haunt my creative process when it comes to creating settings or planning out adventures or campaigns. A place for everything and everything in its place is a fine ethos when it comes to tidying up the house, but when it comes to worldbuilding it is somewhat trickier-- it's fine to utilize it, but sometimes that place has to be well off from the path the adventure is likely to follow.
Let us take the Eberron campaign setting as an example, mainly because it consigns some very popular elements off to certain corners of the world, well away from the continent where most of the "main" action takes place. As obvious as it might seem, an Eberron campaign will probably not involve a lot of giants and dragons, unless one goes out of one's way to allow for trips to Xen'Drik and Argonessen. Something focused on the internal politics of the five nations doesn't generally need them-- and that's perfectly alright, more than that, it's something that one needs to learn and internalize to DM Eberron well. This does not only apply to published settings, of course-- it's okay for things to be out there in your campaign world that the players may never see, may never even hear directly about.
I guess what I'm getting at is that an important skill in GMing, and one I struggle to master, is restraint in the name of focus. This is important in general, but especially so when one is building from the bottom upwards-- a barony or province in the Tiny Bickering Fiefdoms does not need to contain the whole of your Monster Manual, just enough variety to keep things interesting. This is something that to the mind that has already internalized it seems obvious, but something I think it benefits us all to hear now and again.